The present invention relates to radio frequency Tags-Readers for communication, identification and distance measurement and, in particular, it concerns a Media Access Control (MAC) for a system of radio frequency communication between Readers and Tags employing multiple access.
The need for RFID system grows in many areas and for a large diversity of applications, such as asset management and tracking, process control, and asset visibility. It is expected that the use of active RFID devices will reach mass deployments in the future; however at the current time, some limiting factors still exist, among them tag prices, and also some performance limitations. One of the major factors that affects system performance, is the MAC (or the “air interface” protocol).
Prior art MAC schemes include the IEEE 802.11 standard that forms the basis of the WiFi networking which is used all over the world, as well as the IEEE 802.15.4 standard that forms the basis of ZigBee devices. There are also numerous proprietary communication protocols.
The IEEE 802.11 MAC scheme is basically suited for file transfers and not for short packet transfer. While 802.11 offers a very good solution for connection to the Internet for “high-end” mobile devices, such as laptop computers, it is not optimized for very low end devices with hard battery life, cost and size constrains. It does not seem to be the right choice for a scalable active RFID system.
The IEEE 802.15.4 MAC scheme defines two types of Tag devices, namely Reduced Function Devices (RFDs) and Full Function Devices (FFDs), and assumes that devices alternate between “on” and “off” modes. The originator of messages is the RFD. This is one of the reasons that such scheme is not ideal for RFID applications. Another concern is that the system supports Mesh connectivity only between FFDs, while RFDs can connect only to a FFD. Yet another important consideration is that most ZigBee applications deal with a small to medium number of networked devices (e.g. for home control or for automatic meter reading). Further, the 802.15.4 multiple access scheme has a fixed structure.
Radio Frequency IDentification (RFID) is a method of storing and remotely retrieving data using devices called RFID Tags. An RFID Tag is a small device that can be attached to a physical object, animal, or person. Each tag has a unique tag identification number (tag ID). RFID Tags receive and respond to radio-frequency queries from an RFID Reader or from neighboring Tags. An RFID system includes several components including mobile Tags, Tag Readers, and application software. This invention relates to a system using Active Tags (that is, Tags that have an internal power source and can receive and transmit RF signals). Due to the need to obtain a long life span for an active Tag that employs a small battery, the Tag typically alternates between “sleep” and “active” modes. It may typically be in the “active” mode 2% to 10% of the time.
The RFID system enables a query to be received by the mobile Tag and the Tag responds with data. The data is received by an RFID Reader and processed according to the needs of a particular application. The data transmitted by the Tag may provide identification or location information, or specifics about the product tagged, such as type, serial number, price, color, date of purchase.
U.S. Pat. No. 6,992,986, “Integrated self-optimizing multi-parameter and multi-variable point to multipoint communication system” to Reza et al., discloses an adaptive system for point to multipoint wireless communication. The Phy and MAC layers include an adaptive set of parameters. However, in this application, the base station adjusts communication with each customer premises equipment individually, according to the latter capabilities, link condition, etc; while in the current disclosure, the reader sends to the tag population some MAC parameters according to the reader's capabilities and to the assumed group size in each request, it is not required for tag to remember those parameters and it enables the tag to communicate with different readers accordingly.
U.S. Pat. No. 7,257,095, “Power saving via physical layer address filtering in WLANs” to Liu, describes a system and method for saving power in a wireless network, using a physical layer address filtering protocol based on a partial address subset of the complete destination MAC address.
Several patents relate to the usage of relay communication. U.S. Pat. No. 6,785,510 “Routing in a multi-station network” to Larsen, teaches a method of relaying data between mobile stations in a cellular communication system. It is based on a broadcast control channel, as well as on a calling channel (or channels), by which the mobile station obtain connectivity information. This is very different from the system and method of relay that is disclosed in the current disclosure.
U.S. Pat. No. 6,501,582, “Cascaded line-of-sight free-space communication system” to Chiou et al., teaches an optical communication system where repeaters are used in order to relay data between stations.
U.S. Pat. No. 6,987,770 “Frame forwarding in an adaptive network” to Yonge, discloses a scheme which uses payload encapsulation and control information, to allow a frame originated from a first station, and intended for a second station, to be forwarded by an intermediate station. The first station selects the intermediate station based on the data transmission quality that is expected to be achieved.
RFID systems are generally used for a multitude of asset management applications. A warehouse, or a logistic center, may include a large number of tagged objects—say, thousands to hundred of thousands. In such a case there is more than one reader on site, and possibly tens of readers are employed to cover the whole volume. Therefore, it is necessary to “think networking”—namely, to design the communication process and protocol so as to allow efficient access to Tags while taking into account several basic factors and requirements:                (a) The system may include a very large number of tagged objects but should be also efficient if only a small number of Tags is present.        (b) Tags are typically low-cost devices, with limited functionality        (c) Objects dynamically enter and/or leave the customer premises.        (d) Tags alternate between on and off (or active & sleep) modes.        (e) The main mode of communication is between Readers and Tags. In several useful RFID applications, the Reader (also called “Interrogator” in the RFID literature) is the originator of the session.        (f) Having stated the above, the system needs to support some manner of Tag to Tag communication.        (g) The Reader, when acting as a collector node, should be able to maintain a large aggregate data rate, keeping the response time for Tag querying low.        (h) The overall system should be able to support many tagged objects communicating simultaneously.        (i) Unicast, Multicast and Broadcast modes should be supported, as these modes are useful in various workflows required by customers'operations        (j) The MAC should be “power aware”, that is to say able to operate with Tags that alternate between on and off (or active & sleep) modes (even with a low duty cycle).        (k) The MAC should support the basic application needs; the latter include Read tag data, Write tag data, and locate a tagged object.        (l) The communication reliability of the overall system, that is to say communication between the Readers and all Tags currently in the system at any given time, should be very high.        
There is therefore a need for a Media Access Control (MAC) for a system of radio frequency communication between Readers and Tags that address most if not all of the above listed requirements.